Unfortunately, that subset isn't closed or isn't powerful enough to escape primitive recursion; take your pick. Suppose that I'm wrong and it's both closed and

Unfortunately, that subset isn't closed or isn't powerful enough to escape primitive recursion; take your pick. Suppose that I'm wrong and it's both closed and powerful enough. Then, fix an arbitrary Haskell type signature for some function `f`, and consider defining it as `f = undefined` or `f = f` respectively, and work out what still remains in the subset. Indeed, `undefined = undefined` is its typical definition.